1. Technical Field
The invention is a head tracking controller for a floppy disk drive system in which the influence of track radius and the presence of offset error are both eliminated to provide a significantly more accurate head tracking control and a significant advance on the art.
2. Description of Related Art
A hard disk drive, such as the Winchester disk drive, typically includes a plurality of hard disks rigidly held together and rotated simultaneously by the disk drive and contacted by a corresponding plurality of read/write heads rigidly coupled together. One of the many disk surfaces is dedicated to prerecorded tracking signals. That head associated with the dedicated disk surface picks up the prerecorded tracking signals and sends them to a head servo, which uses these signals to control the radial position of all the heads. The purpose of the head servo is to keep all the heads in the center of the desired disk track.
3. Discussion related to the Problem
Floppy disk drives present more difficult tracking control problems, because the disks are not rigid and therefore cannot be rigidly joined together as a plurality of disks in which one of the disk surfaces is dedicated to generating constant tracking error signals. Thus, the head tracking control system for hard disk drives is not useful for floppy disks. Instead, each floppy disk must be operated separately and independently, with its own prerecorded tracking error signals sharing space with the recorded user data. Because the prerecorded tracking error signals must share space with the stored data, they are preferably limited to short bursts so that their processing is more critical than the processing of the constant tracking signal characteristic of a hard disk drive.
Tracking signals are typically prerecorded on both the right and left side of each disk track. The head picks up both the right and left signals, and the two are separated and subtracted from one another, to generate a head tracking position error signal. The presence of two (right and left) simultaneous tracking signals rquires two channels, or sets of circuits, to perform the separation and subtraction steps. The two parallel channels are characterized by an offset error between them (due to unavoidable component differences) and this offset error is unavoidably introduced into the error computation process.
The prerecorded tracking signal, as sensed by the head, has a diffferent waveform--and therefore a different energy content--at different disk radii or tracks. (This is because the bit density on the disk is inversely proportional to the track radius). As a result, using the prerecorded right-hand and left-hand tracking signals sensed by the head to compute head tracking position error produces different error signals at different disk radii for the same tracking error. Thus, there is a built-in radial bias error in the tracking control system.
In summary, floppy disk drives are characterized by limited space for prerecording tracking signal, a built-in offset error and a built-in radial bias error. In the prior art, these undesirable characteristics have remained an impediment to improving disk drive accuracy and therefore constitute a fundamental limitation.